Important components of direct solar based nanowire enabled chemical processing and electrochemical systems are a high efficiency and highly stable photocathode and 2-photon dual electrodes. The former enables photo-excited electrons lead to hydrogen (H2) generation whereas the later with complementary energy bandgap photoanode and photocathode enables high efficiency, unassisted solar-driven water splitting.
Photocathode: Within the prior art extensive studies have been performed to develop photocathodes that can simultaneously absorb a large part of the solar spectrum and yield efficient charge carrier separation and proton reduction. However, in order to achieve this, the semiconductor light absorber should have a conduction band minimum (CBM) more negative than that required for hydrogen evolution reaction (HER), which is 4.44 eV below the vacuum level in solutions with pH=0. This requirement limits the choice of high efficiency semiconductor photocathodes primarily to silicon (Si) and a few III-V semiconductor materials, these including gallium phosphide (GaP), indium phosphide (InP) and their associated alloys. Various HER catalysts or protection layers integrated with Si exhibited improved performance compared to platinized p-Si photocathodes. Some other materials have also been studied, but are often limited by either rapid degradation or very poor absorption of visible light.
To effectively utilize photons within a wide range of the solar spectrum, a dual light absorber with a narrow bandgap material like Si at the bottom and direct wide-bandgap materials on top can provide energetic electrons for H2 production. Accordingly, researchers have demonstrated that various heterostructures integrated with Si can exhibit improved performance compared with catalyst coated Si, e.g. platinized p.Si photocathodes. However, the design and performance of such multi-junction devices is limited by the current matching related issues between the two absorbers, because the carrier collection and extraction is only available on the front surfaces. For such photoelectrodes consisting of dual or multiple light absorbers, although the required external bias can be reduced, the photocurrent density is ultimately limited by the light absorber that provides the smaller maximum photocurrent density. Recently, the use of 1D nanostructures, such as nanowires, has been intensively studied, which can enable highly efficient carrier extraction and proton reduction on the large area lateral surfaces. To date, however, there have been no reports on such nanowire-based monolithically integrated multi-junction photoelectrodes.
Accordingly, it would be beneficial to leverage the high surface areas and self-contained conversion of direct solar illuminated hydrogen generation from such nanowires with multiple junctions for broad solar spectrum absorption by providing monolithically integrated multi-junction photocathodes.
Dual-Photoelectrode: Among the various photoelectrochemical (PEC) designs, a P-N dual-photoelectrode device, also commonly referred to as a photochemical diode, promises significant performance advantages and cost benefits. Such a 2-photon dual-electrode system can be implemented with two semiconductors connected back-to-back in tandem, forming the top and bottom photoelectrodes. In this scheme, minority carriers are driven to the semiconductor/liquid junction to perform oxidation/reduction reactions, while majority carriers recombine at the photocathode/anode interface. In a P-N dual-photoelectrode system, the electron-hole pair chemical potential can be made equal to, or greater than the largest band gap of the two semiconductors. As such, it can address the critical photovoltage bottleneck of a single-photoelectrode system, thereby leading to unassisted, solar-driven water splitting and hydrogen generation. Another fundamental advantage of the dual-photoelectrode system lies in that the two semiconductors can be designed to be complementary light absorbers. By separately optimizing the bandgap of the light absorption layers, the device efficiency can reach a theoretical maximum of 40%, and 29.7% considering reasonable energy loss processes. The P-N dual-photoelectrodes also offer several important advantages compared to photovoltaic-PEC (PV-PEC) and photovoltaic-electrolysis (PV-EL) technologies, including much simpler fabrication process, significantly reduced operation voltage, and potentially higher efficiency. Further, the simple 2-photon P-N dual-photoelectrode, PV-PEC or PV-EL photoelectodes can also be paired in a dual configuration to achieve improved solar-to-hydrogen efficiency.
In spite of their promise, conventional 2-photon tandem photoelectrodes generally exhibit very poor performance, with the commonly reported efficiency in the range of ˜0.1%, which is significantly smaller than the best reported single photoelectrode (˜1.8%) and PV-PEC devices (12.4%). However, within the prior art a specific instance of higher efficiency (˜0.9%) has been demonstrated with haematite photoanode and amorphous Si photocathode with NiFeOX and TiO2/Pt overlayers. In prior art tandem dual-photoelectrodes, dissimilar materials were used to provide complementary bandgaps and the resulting photovoltage small, limited by the material quality and incompatibility. Moreover, the device efficiency was severely compromised by the poor interfacial properties and, in many cases, by the performance of the Ohmic contact or tunnel junction connecting the electrodes. Further, due to the dissimilar material properties, the optimum performance of the two electrodes may require the use of different electrolytes. For example, prior art n-WO3/p-Si dual tandem photoelectrodes only showed modestly enhanced photovoltage but with tremendous compromises in photocurrent and efficiency.
Accordingly, it would be beneficial to provide nanowire based dual-photoelectrode systems operable in acidic electrolyte which, together with a parallel illumination scheme, can fundamentally address these critical challenges. It would be further beneficial for these nanowire based dual-photoelectrode systems to exploit a semiconductor material family that can be tuned across the solar spectrum, can be doped both p-type and n-type and supported large current conduction.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.